1. Field of the Invention
The present invention relates to an air cleaner of automobile engine, specifically to structure of an air cleaner case in the case the intake air flow of the engine is measured in a zone downstream from the air cleaner.
2. Description of the Related Art
In an automobile engine, it has become necessary to control the flow of EGR (Exhaust Gas Recirculation) gas in correspondence with the intake air flow in order to meet upcoming regulation of exhaust gas. To deal with this, an air flow sensor is provided downstream of the air cleaner to measure the intake air flow.
Turbulence and drift in the intake air flow changes when the shape of the air cleaner case and duct are changed, and noise occurs in the output of the flow sensor, causing inaccurate measurement of the intake air flow.
There is a case the air cleaner element is different according to the use of the engine. A single type element is used for the air cleaner of a cargo truck and bus, and a double type element is used for the air cleaner of a dump truck which runs many hours in dusty districts. Air cleaner cases and intake air flow measuring apparatuses of the same type are used in many of these cases. FIG. 4 and FIG. 5 show respectively when a single type element and a double type element is used in an air cleaner case of the same design.
In FIG. 4 and FIG. 5, reference numeral 1 is an air cleaner case, and 2′ is an outlet duct extending outward from the air cleaner case. The duct 2′ is connected to the intake pipe of an engine not shown in the drawings. Reference numeral 3 is a single type element, 4(4a and 4b) is a double type element, and 5 is a case cover to be attached to the air cleaner case 1. The air cleaner element 3 and 4 are formed to be cylindrical and held in the air cleaner case 1 by means of the case cover 5.
An air flow sensor 6 is located in the duct 2′ downstream from the air cleaner case, i.e. at a position nearer to said intake pipe not shown in the drawing. Air is sucked through an inlet duct 7 into the pre-clean room 8 in the air cleaner case 1, flows through the element 3 or 4 to the post-clean room 9 in the air cleaner case 1, and flows toward the said intake pipe through the outlet duct 2′.
In the case the air cleaner case 1 of the same design is used for the single type element 3 and for the double type element 4, a radial step G1 is caused between the inner surface of the duct 2′ and that of the element 3 when the single type element 3 is used as shown in FIG. 4, and a radial step G2 is caused between them when the double type element 4 is used as shown in FIG. 5. When there is a radial step between the inner surface of the duct 2′ and that of the element like these, turbulence is generated in the air flow at the radial step.
The turbulence propagates toward downstream, which does not disappear until it reaches fairly far from the radial step.
However, it is impossible in the case of an automobile engine to secure distance A between the entrance to the duct 2′ and the center 6a of the air flow sensor 6 long enough for the turbulence to disappear because of restricted engine mounting space, and it is unavoidable to locate the air flow sensor 6 at a zone where measurements is influenced by the turbulence.
There has been disclosed in JP9-269251A an air flow measuring device for reducing noise in the output of an air flow sensor, in which a secondary air flow passage less influenced by disturbance and drift in the intake air flow is formed in addition to a main air passage and the air flow sensor is located in the secondary air flow passage.
However, flow resistance inevitably increases with the flow measuring device having such a secondary flow passage.
Concerning the accuracy of flow measurements, it is possible to correct the measurements on the basis of experimental relation between the measured and actual flow rate obtained beforehand. However, in this case, two control units of which one is imputed with a calibration formula for a single type element and the other is inputted with a calibration formula for a double type element must be prepared, which causes an increase in cost and brings about additional parts management work.